Transmission network



April 14, 1936. w 5|X I 2,037,546

TRANSMISSION NETWORK Filed July 17, 1933 AQEOI/A-WCY i sou/2c: i

INVENTOR W/LLEM S/X g m I ATTORNEY Patented Apr. 14', 1936 PATENT OFFICE 2,037,540 I TRANSMISSION NETWORK Willem Six, Eindhoven, Netherlands, assignor to Radio Corporation of America, a corporation of Delaware Application July 17, 1933, Serial No. 680,780

In Germany July 16, 1932 9Claims.

This invention relates to an arrangement for transmitting music or speech by electrical means through a network of mains in which, the voltage of the energy supplied by the output ampli- 5 fiers is supplied to the subscribers through at least two transformers. The central point from which the energy supplied from the output amplifiers is supplied to the feeders is not connected with respect to direct current to the consumer's mains, so that for measuring the insulation resistance or the load of these mains by a direct current, pilot wires have to be connected to the secondary of the second transformer. The more distant this transformer from the central station, the more will this disadvantage be pronounced.

An object of the present invention is accordingly to provide a transmission network in which such pilot wires will not be required and in which the loudspeakers and mains of the consumers may be tested by a direct current sent over the main supply mains.

A further object of the invention is to provide a novel form of conductive connection between the primary and secondary windings of a transformer.

A still further object of the invention is to provide a novel arrangement for balancing the windings of an audio frequency transformer with respect to ground so that a balanced system is ob- 30 tained. Other objects will become apparent to those skilled in the art as the description thereof proceeds.

For a better understanding of the invention reference is made to the accompanying drawing in which, a

Fig. 1 is a schematic circuit diagram of a centralized audio frequency distribution system embodying the invention; and,

Fig. 2 is a schematic diagram showing a modifled arrangement of the transformers of Fig. 1.

One example of a system in which at least two transformers are used is a system in which the voltage supplied from the output amplifier isstepped down in two or more stages by trans- 45 partly after one or more of the feeders starting from the amplifiers. The use of such a system is based upon the following considerations.

If music or speech is to be distributed by elecers, the branches of this network must have such a cross section that even the potential loss occuring at the most remote point remains within admissible limits. If the network is extensive and if many subscribers are connected andthe voltformers which are situated partly before and trical means from a central station through feedage to be transmitted is comparatively low the cross sections of the main lines will consequently be large which renders the network more expensive. If, however, a higher voltage is passed through comparatively long feeders towards fav- 5 orably situated distributing points in which it is stepped down and passed on to the subscribers, a considerably cheaper supply system suifices at the same loss of maximum voltage. As the energy to be distributed is usually supplied at the central or generating station from the output amplifier at a voltage which is so many times larger than that desired by the subscribers, that the capacitative losses in the feeding cables would increase to an excessive extent if the currents were sup- I plied to the distributing points. at this high voltage, a compromise is made between these two considerations and the voltage supplied from the output amplifieris stepped down both at the beginning and at the end of the feeder line.

The invention has for its object to overcome the difiiculty referred to in the preamble with respect to the use of pilot wires, particularlywith the last mentioned system, and consists in that both the primary and the secondary of the sec- 0nd and/or any further transformers through which the voltage supplied from the output amplifier is supplied to the subscribers, are divided into two parts, the parts of each winding being inter-connected unilaterally through a condenser common to the two windings.

As shown in the drawing a centralized radio system is shown comprising a central station generally denoted by the reference letter A. At said station voice or music modulated currents from any suitable source I, which may be a broadcast radio receiver of any well known type or other source of audio frequencies, are amplified by a power vacuum tube amplifier 2. The output of amplifier 2 is connected to a pair of' 40 long leads or conductors 'l by the primary 5 and secondary 6 ofan audio transformer 4. The leads I conduct the audio currents to a plurality of loudspeakers or other sound reproducing devices |9 which may be located in the different rooms of a building such as a hotel or apartment house.

Inserted in line i are the primaries of audio transformers 8 which are preferably arranged to step down the applied voltage as is also the trans- 5 former l. The primaries of transformers 8 are subdivided into'two equal windings or sections 9 and I0, the ends of the sections being connected to the upper and lower plates l5 and ll of condensers II; The secondaries of 8 are similarly subdivided into two equal sections or windings I 2 and I3 the ends of which are connected to plates 15 and I8 as shown. It will be understood that the sections of the primary windings of 8 are inductively connected to the sections of the secondary windings. As shown the voice or operating coils l8 of the speakers l9 are connected across the output terminals ll of the transformer secondaries. It will be understood that the transformers 8 are located near or adjacent to their respective speakers l9, each transformer and its speaker being located, for example, in the same apartment and at a remote distance from central station A.

According to my invention the impedance of each condenser I"! at audio frequencies is much lower than the load impedance of the secondary of the transformer 8. With this construction the currents to be transmitted by the transformer do not bring about any substantial potential difference between the terminals or plates of condenser l1.- At the same time the condenser plates provide conductive connections between the primary and secondaries of 8,'so that a direct current path is provided between the central station A where the leads I terminate and the energizing coils 18 of the utilization devices I9.

This direct current path may be utilized for testing the connections of transformers 8 and coils l8 by any known direct current testing method and instrument at station A and without the use of extra pilot wires running from the station to the several devices l8. While I have shown only three speakers connected to line I, it will be understood that a considerably larger number may be used.

In one embodiment of an arrangement according to the invention in which the load impedance of the secondary of the transformer consisted of 300 loud-speakers having each an impedance of about 10,000 ohms at 800 cycles a condenser H of about 50 microfarads was used.

For the purpose of balancing the network, the middle point of the primary of any of the transformers 8 may be connected to earth through two series connected condensers 20 and 2| which are connected in parallel with the condenser I1 and whose connecting point is earthed by a lead 22 as shown in Fig.- 2. The capacity of these condensers should be such that the impedance formed by a part of the winding of the transformer 8 and the particular condenser 20 or 2| between each of the lines and the earth is very low relatively to the insulation resistance and to the impedance due to the capacity of each of the lines relatively to earth. In one embodiment of the arrangement according to' the invention two condensers of about 5 microfarads each were used.

Having described two forms of my invention it will be understood that it is not limited to the forms herein shown, but is to be limited only by the scope of the following claims when considered in connection with the prior art.

What I claim is:

1. In combination in an electrical distribution system, a line terminating at a central station,

-mea.ns for impressing a band of frequencies across the terminals of said line at said central station, a plurality of utilization devices located at separated points along said line at a distance from said central station and a plurality of transformers arranged to connect said devices to said line, one or more of said transformers having a primary and a secondary winding and two conductors connecting said windings, said conductors being spaced apart to form an electrical condenser.

2. The combination defined in the preceding claim in which said condenser has a low impedance value over the band of frequencies to be transmitted.

3. In an electrical distribution system, the combination of a transmission line, means for impressing an audio frequency voltage across one end of said transmission line, a sound reproducer located near the other end of said line and an iron core transformer arranged to connect said sound reproducer to said line, said transformer having a primary formed of two substantially equal winding sections and plates of large area to which the ends of said sections are connected, said plates being slightly separated to thereby form a condenser of large capacity value.

4. In an electrical distribution system, the combination of a long transmission line, means for impressing potentials which vary at audio frequencies across one end of said transmission line,

a sound reproducer located near the other end of said line and including a voice coil, a transformer having a two coil primary and a two coil secondary winding the top and bottom terminals of said primary coils being connected to the ends of said line, and the top and bottom terminals of said secondary coils being connected to said voice coil, and two condenser plates to one of which the middle terminals of a primary and a divided primary and secondary windings connected respectively to said line and said utilization device a two plate condenser one of whose plates serves to conductively connect one primary to one secondary winding section and whose other plate serves to conductively connect the other primary and secondary winding section, two condensers having a common inner terminal, means for connecting the outer terminals of said condensers to the respective plates of said first named condenser and a ground connection to the common inner terminal of said two condensers.

6. The combination defined in the preceding claim in which the capacity value of said first named condenser is substantially greater than that of either of said last named two condensers.

7. In combination in an electrical distribution system, a two wire transmission line, means for impressing a band of currents of different frequencies across one end of said line, a utilization devicehaving an energizing winding, and means for conductively connecting said winding to said transmission line comprising a transformer having a divided primary and a divided secondary winding and a pair of spaced apart plates each connected to an intermediate point of said primary and secondary windings said plates being arranged to provide a path for direct current between said transmission line and enerz ng winding.

8. The combination defined in the preceding claim in which said plates are of suflicient area to form a condenser having a large capacity value.

9. In combination, a transmission line, means for impressing audio frequency voltages across one end of said line, a utilization device having an energizing-coil, a transformer having primary and secondary windings connected respectively to said line and said coil, a two plate condenser each plate oi which metallicaily connects an intermediate point of said primary to an intermediate point of said secondary winding and two condensers connected in series across the plates of said first named condenser, the eiiective capacity of said last named condensers being greater than that of said first named condenser. 

